DNA is constantly being damaged by a variety of cellular and environmental agents. In order to prevent DNA mutagenesis, cells have evolved a variety of effective repair mechanisms to correct damaged DNA. However, during replication, not all DNA damage is repaired, rather, damage instead is tolerated by a group of translesion DNA polymerases such as Polymerase eta and Polymerase zeta (Pol C). Evidence in yeast indicates that the catalytic subunit of Pol delta, Rev3, is capable of extending from misincorporated nucleotide templates and is responsible for both damage-induced and spontaneous mutagenesis. Very little is known about the enzyme in mammalian cells. Data suggests that loss of vertebrate Rev3 leads to increased genomic instability and greater susceptibility to the effects of DNA damage. We plan to study the contribution of Rev3 in damage tolerance when wild-type and Rev3 -/- cells are treated with various DNA damaging agents. Furthermore, we will attempt to determine if the polymerase activity is important in damage bypass and whether truncated portions of the protein is sufficient to restore damage tolerance. Finally, data indicates that Rev3 mRNA is regulated at various levels. Mammalian Rev3 protein expression patterns are unknown. We plan to generate an antibody in order to study tissue specific protein expression and whether Rev3 is upregulated during periods of genotoxic stress via immunoblot analysis.